My brother in law has a serious medical condition that makes it almost impossible to control his weight. At one time he was probably about 400 lbs (at about 5 ft 2) My MIL makes sure to call the airlines before he flies to discuss his condition and special needs. Several airlines have given him an automatic upgrade to first class (w/ the larger seats). Other airlines suggested that she book a middle seat for him and fly at off peak times, which would reduce the likelihood of that seat being filled. Either way she was ALWAYS prepared to pay more to ensure that the seat next to him was empty, because she wanted to ensure his comfort and was concerned about the person would 'might' have to sit next to him.
Bottom line is this, most folks KNOW when they don't fit in an airline seat. Quiet as it is kept, the airlines have been shrinking seats for a while to get more people on a flight and therefor increase revenue. If you know you are going to be uncomfortable AND make the people next to you uncomfortable be courteous enough to make some arrangement before had.
I understand SW's point, but I too wonder how they will enforce this. I have visions of those little bins that they use to see if your luggage will fit in the over head compartment: "Sir, could you have a seat here so we can see if you will fit."
Another question....what about extra tall people? My husband is pretty tall and is extremely uncomfortable in coach, especially when he has to sit next to the window. He also makes sure to try to get the seats with more leg room when he flies (first seat in coach, emergency exits) and is very conscious about poking the person in front of him with his knees. That is the most annoying thing!!